Production of three dimensional sintered bodies is traditionally a complex and costly process, e.g. porous bodies. Porous bodies are used for many different technical applications in filtration of gases and liquids, fluidising processes, separation and catalysing. One important application is in filtration of pulps. The shape of the porous body or mould is in the latter case quite complicated. However, the shape of filters for gases and liquids is commonly quite simple due to the inherent manufacturing problems in producing porous bodies having a more complex configuration. Furthermore the production series may be of relative small size, why a low production cost of the mould is an advantage, as also fast and cost effective way of manufacturing it. Another aspect is the internal structure of such products, e.g. drainage properties.
There are some key properties of a filter that have large importance on the requested functionality at the filtering operation. Two of these properties, i.e. high yield at the filtering operation and low pressure drop over the filter, are very often contradictory. High yield of the substance to be taken away by the filtering operation e.g. fibres in a pulp is achieved by using a filter with a fine porous structure. A fine porous structure means in this case that the channels formed by the pores are small in size. However, the pressure drop over the filter increases with decreasing size of the pore channels. On the other hand as mentioned above, a fine structure is often desired to obtain high yield and some times also to facilitate providing a body having a smooth surface structure.
It is well known that a fine pore structure can be achieved by manufacturing the porous body from fine particles or fibres and obviously a coarse pore structure by using coarse particles or fibres in the manufacturing process of the filter.
One rather recent method for producing three-dimensional objects, e.g. to provide filters with a graduated structure, is the WPS-process (Wet Powder Spraying Process). In this process a mixture containing sinter able powder is sprayed on to a supporting body, where after the binding agent of the mixture is decomposed in a first step and the actual sintering is performed in a second step. In US 2004/0050773 there is presented such a process intended for the production of at least two layers of different pore size.
From US 2002/0195188 there is known a similar but somewhat modified process wherein the mixture contains pore forming material containing particles with predetermined size or size distribution. Further, there is also known a process wherein centrifugal forces are used to obtain a graduated structure of a three-dimensional sintered body. As is evident, all of these processes need sophisticated arrangements/apparatuses to achieve a desired result, which leads to a relatively high cost of the final product.
From EP0559490 and EP0559491 there is known a further process for producing a three dimensional body, e.g. a pulp moulding die, preferably comprising glass beads to form a porous structure is presented, which also mentions that sintered particles can be used. The production technology described therein is however complicated.